Codicology, or the study of manuscripts as material objects, is a cornerstone of historical and literary research. The emerging field of ‘biocodicology’, though, crosses disciplinary boundaries by reading books and documents made out of animal skin as biological items. Indeed, by analysing the proteins found in early modern legal records, a team of researchers from the universities of Exeter, York, and Cambridge has been able to prove that people in early modern Britain preferred to write their documents on sheepskin – a choice of material that may have been strategically motivated by a desire to stop the items being fraudulently modified.
The study of proteins, or proteomics, is fast becoming a standard method of archaeological investigation, and ‘Science Notes’ has previously explored the technique’s capacity to determine the health of past cultures via analysis of human bone (CA 353) and dental plaque (CA 338). In applying proteomic methods to early modern documents, however, Dr Sean Doherty (University of Exeter) and his colleagues have been able to differentiate between multiple possible animal species used in the production of legal deeds.
Scholars have suspected that medieval and early modern documents tended to be written on sheepskin, rather than on expensive calfskin (known as ‘vellum’), which was reserved for higher-quality books like the York Gospels (CA 333). But the researchers found that there was little concrete data relating to the species of animal used in the production of pre-modern legal deeds. These have usually been categorised subjectively, by visual analysis of hair fibres and follicle patterns, and variously catalogued in archives and libraries as ‘vellum’ or ‘parchment’ (generally meaning sheep- or goatskin), with some items being labelled even more indeterminately as ‘animal membrane’ or simply misidentified. In order to gather more conclusive data, Sean’s team examined 645 pages belonging to 477 legal deeds linked to English, Scottish, and Welsh property agreements made between 1499 and 1969.
The research team took small (2mm²) skin samples from each page, and each sample was chemically treated to allow collagen to be extracted from the skin. Although all animal skin contains collagen (a protein formed of a combination of subunits called peptides), the mix of peptides in each species’ collagen is unique. The team therefore used mass spectrometry techniques (ZooMS) to determine the molecular weights of the individual peptides in each sample, which permitted the generation of a ‘peptide mass fingerprint’ for each. This was then cross-referenced with known reference data to identify the species present in each deed. The results, published earlier this year in Heritage Science, showed that 622 samples (96.4%) contained sheep protein. The authors argue that this indicated an overwhelming use of sheepskin in early modern British legal deeds, which relates not only to an abundance of sheep and to the low cost of sheepskin parchment at this time, but also to the unique biological structure of that parchment, which is made from an inner layer of skin called the dermis.
Parchment production involves soaking an animal skin in a lime solution to remove the fibres, as well as the epidermis, or outer layer of skin. The same process draws out fat from the skin, which is scraped away along with excess flesh. The pelt is then stretched, dried, smoothed, and cut to create a writing surface. The researchers noted that sheepskin parchment was lauded by pre-modern writers for its ability to make fraudulent textual erasure visible, a quality the team has attributed to the material’s high fat content (30-50%) compared to cattleskin (2-3%) and goatskin (3-10%).
In fact, because the dermis itself consists of two layers, the papillary and the reticular dermises, a relatively large void can develop between these layers in sheepskin when fat is removed during the liming process, leading to visible disintegration of the document if it is tampered with. ‘Removing fat during the parchment-making process can cause the layers within sheepskins to separate more easily than those of other animals,’ Sean explained. ‘To make fraudulent changes to documents after signing, the original text would have to be scraped off. This could cause the layers within sheepskin parchment to separate and leave a visible mark on the document, resulting in the fraud being easily detectable.’
The study is yet another demonstration of the importance of proteomics in the study of the past. The paper is available online at https://doi.org/10.1186/s40494-021-00503-6.